The present invention relates to an image processing system for an endoscope which allows stereoscopic viewing using a monocular lens, an endoscope system which performs image processing of an image from the endoscope to visually display the image, and a method of controlling the endoscope.
Since an image obtained by a conventional monocular endoscope does not allow easy recognition of, e.g., the relative distance between a plurality of objects to be observed due to its insufficient information in the depth direction, operation errors often occur in an operation while observing the image via the endoscope. Since an operator must concentrate in order to avoid such operation errors, he or she suffers considerable fatigue both mentally and physically.
In order to solve the above-mentioned problems, stereoscopic endoscopes that allow stereoscopic viewing have been developed. In most stereoscopic endoscopes, stereoscopic images are fetched using binocular observation lenses, and are transmitted to a stereoscopic image output device via two different optical systems, thus displaying the stereoscopic images.
However, in the endoscope that allows stereoscopic viewing, since the binocular optical lenses and the two different optical systems are juxtaposed, the outer diameter of the stereoscopic endoscope increases, thus disturbing a decrease in diameter of the endoscope.
As a method of solving the problem of an increase in outer diameter, Japanese Patent publication for opposition No. 5-67931 discloses a method of transmitting parallax images within a single optical path by using a pair of polarization filters which have different azimuth angles of polarization. More specifically, the pair of polarization filters are attached to a pair of observing lenses respectively in this conventional endoscope. The pair of polarization filters generate a pair of polarized images having a parallax. A half prism combines the pair of polarized images into a single optical path to propagate within the endoscope by using polarization preservation fiber. The two polarized light images are image-sensed time-divisionally or time-parallelly to be converted into video output signals, which are input to a stereoscopic image display device. An observer observes an image on the display device as a stereoscopic image.
Two problems are posed when an image obtained by the monocular observation lens is converted into two polarized light images using the binocular polarization filters and the two polarized light images are guided in one light transmission system. That is, first, the axis of polarization of guided light shifts, and second, the two polarized light images suffer intensity unbalance.
The shift of the axis of polarization of guided light means that the plane of polarization of given linearly polarized light (e.g., polarized light having only an X-axis component but no Y-axis component) gradually shifts during propagation. As a result of the shift, the linearly polarized light undesirably acquires a Y-axis component, and consequently, crosstalk occurs in stereoscopic viewing. In the conventional system, the problem of the shift is solved by using expensive light transmission means such as a polarization holding fiber.
The intensity unbalance between two polarized light images as the second problem occurs for the following reason. That is, object light from an observation object is naturally polarized by the surface state or material of the object. The stereoscopic endoscope using the monocular observation lens can obtain a satisfactory stereoscopic image under the condition that the two polarized light components in light incident on the observation lens have equal intensities. However, as described above, when the polarization state of the object light itself is offset (for example, if the two axes of polarization are represented by X and Y, the intensity of a polarized light component X is higher than that of the polarized light component Y), the incident light itself onto the observation lens has polarization characteristics. As a result, when the pair of polarization filters are set in advance so that one of the filters transmits only polarized light X (left-eye image light) and the other filter transmit only polarized light Y (right-eye image light), the two polarized light images obtained from these filters suffer intensity unbalance (the intensity of the left-eye image light is higher than that of the right-eye image light).